I came across this sentence but the “行きたくなった” is throwing me off. The full sentence is 授業中に、トイレに行きたくなった" and the translation says “During class, I had to go to the bathroom”
My question is shouldn’t it be “行きたかった”？ But that would be more like “wanted to go to…” Even if it was a negation it would be “行きたくなかった” right? Or is it a completely different grammar point? 
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It’s based on the grammar point for なる, meaning “to become.”
Step by step:
行く - to go
行きたい - want to go
行きたくなる - to become wanting-to-go. This might be a decision, or it may be just some change of state, like becoming taller.
行きたくなった - did become wanting-to-go
Since this is about using the bathroom, we usually say “needed to go” rather than “wanted to go”. Generally, the school bathroom isn’t The Place You Want To Be In, at least in English.
However, if you really need a toilet, the bathroom is the place you want to go.
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Hey there!
Actually, it’s just a case of a minor misreading. The literal translation of “行きたくなった” is “It became that I wanted to go (to the bathroom).” The なった here is the casual past form of なる, and since 行きたい functions as an い-adjective, it becomes 行きたく before なる. “行きたかった” means “I wanted to go (to the bathroom)” which is close but not quite the same. You’re right about the negative-past form though!
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